1. Technical Field
The present invention relates to an electric power steering apparatus for assisting steering force by power of a motor.
2. Background Art
FIG. 18 is a control block diagram of a conventional electric power steering apparatus disclosed in, for example, Japanese Patent Publication (unexamined) No. 17424/1995.
In FIG. 18, reference numeral 1 is a torque sensor for outputting. a steering torque of a driver as a steering torque signal Vt, and numeral 2 is a speed sensor for outputting a speed of a vehicle as a speed signal Vs. Numeral 3 is a CPU for outputting a motor drive signal Sm corresponding to an inputted signal such as the steering torque signal Vt and detecting failure in the electric power steering apparatus, and numeral 4 is drive inhibiting means for judging a direction of inhibiting drive of the motor corresponding to the steering torque signal Vt and outputting a rightward drive inhibition signal InhR and a leftward drive inhibition signal InhL. Numeral 5 is motor driving means outputting a motor current Im corresponding to the value of the motor drive signal Sm, the right drive inhibition signal InhR, and the left drive inhibition signal InhL, and numeral 6 is a motor for generating a steering assist force corresponding to the motor current Im.
FIG. 19 is a diagram showing the mentioned conventional electric power steering apparatus in the form of whole constitution.
In FIG. 19, numerals 1, 2, and 6 are the same as those in FIG. 18. Numeral 7 is a steering wheel, numeral 8 is a steering shaft, numeral 9 is a reduction gear for transmitting the outputted torque of the motor 6 to the steering shaft 8, numeral 10 is a controller for driving the motor 6 on the basis of signals inputted from the torque sensor 1 and the speed sensor 2, and numeral 11 is a battery for serving as a power source of the controller 10.
FIG. 20 is a diagram showing a characteristic of the drive inhibiting means in the conventional electric power steering apparatus.
In FIG. 20, in a case that the steering torque signal Vt is larger than a threshold value Tth1, i.e., in the case that steering to the right is conducted, the rightward drive inhibition signal InhR is authorized, while the leftward drive inhibition signal InhL is inhibited. In the case that the steering torque signal Vt is smaller than a threshold value xe2x88x92Tth1, i.e., in the case that steering to the left is conducted, the rightwad drive inhibition signal InhR is inhibited and the leftward drive inhibition signal InhL is authorized. It is established that the drive inhibition signals InhR and InhL are both inhibited in the case that the steering torque signal Vt is not more than the threshold value Tth1 and not less than the threshold value xe2x88x92Tth1, i.e., in the case that steering is not operated or steering is operated with a very small steering force.
FIG. 21 is a table showing the conditions of outputting a motor current in the conventional electric power steering apparatus.
FIG. 22 is a flow diagram showing a process of judging whether or not there is any failure in the drive inhibiting means of the conventional electric power steering apparatus.
FIG. 23 is a graph showing operation of the controller when it is judged that there is any failure in the drive inhibiting means of the conventional electric power steering apparatus.
Described below is operation of the foregoing conventional electric power steering apparatus.
The torque sensor 1 determines the steering torque of the driver and outputs the steering torque signal Vt. The speed sensor 2 determines the speed of the vehicle and outputs the speed signal Vs. The CPU 3 outputs the motor drive signal Sm of a predetermined characteristic corresponding to the steering torque signal Vt and the speed signal Vs. The drive inhibiting means 4 outputs the drive inhibition signals InhR and InhL of predetermined characteristics corresponding to the steering torque signal Vt. The motor driving means 5 outputs the motor current Im driving the motor 6 corresponding to the motor drive signal Sm and the drive inhibition signals InhR and InhL.
As shown in FIG. 21, the motor driving means 5 outputs the motor current Im corresponding to the motor drive signal Sm in the case that the driving direction of the motor drive signal Sm is not inhibited by the drive inhibition signal InhR or InhL. On the dother hand, in the case that the driving direction is inhibited by the drive inhibition signal InhR or InhL, the motor current Im is set to 0. The motor 6 outputs the steering assist force corresponding to the motor current Im, thereby reducing the steering force of the driver.
In the case that the CPU 3 fails and is in a failure mode wherein the motor drive signal Sm is outputted regardless of the steering torque signal Vt, the output of the motor 6 in the direction opposite to the steering torque signal Vt is inhibited when the driver is steering the vehicle. Therefore it is possible to prevent increase in steering force of the driver caused by the motor 6. Moreover, when the driver is not steering the vehicle, the output of the motor 6 in both directions is inhibited, and therefore it is possible to prevent undesirable self-turning of the steering.
As shown in FIG. 22, whether or not there is any failure in the drive inhibiting means 4 is judged by comparing the state of the steering torque signal Vt with that of the drive inhibition signals InhR and InhL.
In FIG. 22, in step S1, the absolute value of the steering torque signal Vt and the threshold value Tth1 are compared with each other, and when it is judged that the absolute value of the steering torque signal Vt is not larger than the threshold value Tth1, the drive inhibition signals InhR and InhL are monitored in step S2 and step S3. If at least one of the drive inhibition signals InhR and InhL is authorized, the process proceeds to step S7, where it is judged that there is any failure in the drive inhibiting means 4.
In the case that it is judged in the step S1 that the absolute value of the steering torque signal Vt is larger than the threshold value Tth1, the process proceeds to step S4 to judge the direction of the steering torque signal Vt. If the direction is judged rightward, the process proceeds to step S5 to monitor the leftward drive inhibition signal InhL. If the signal is authorized, the process proceeds to step S7 to judge that there is any failure in the drive inhibiting means 4. In the case that the direction of the steering torque signal Vt is judged leftward, the process proceeds to step S6 to monitor the rightward drive inhibition signal InhR. If the signal is authorized, the process proceeds to the step S7 to judge that there is any failure in the drive inhibiting means 4.
As a result of the foregoing processing, if it is judged that there is any failure, occurrence of the failure is definitely concluded, and the motor current Im is set to 0 in order to interrupt the drive of the motor 6 as shown in FIG. 23.
As described above, the electric power steering apparatus for reducing the steering torque of the driver is achieved in the conventional apparatus by driving the motor 6 corresponding to the steering force of the driver and generating the steering assist force.
In the case that the CPU 3 fails and outputs the motor drive signal Sm regardless of the steering torque signal Vt, the drive inhibiting means 4 restricts the output of the motor 6, whereby it is possible to prevent undesirable self-turning of the steering.
In the case that the CPU 3 judges that the drive inhibiting means 4 fails and can not restrict the output of the motor 6, the drive of the motor 6 is interrupted, whereby safety is assured against a failure mode in which the CPU 3 fails after the failure of the drive inhibiting means 4.
FIG. 24 is a control block diagram showing a further conventional electric power steering apparatus disclosed in, for example, Japanese Patent Publication (unexamined) No. 315330/1997.
In FIG. 24, numeral 12 is drive inhibiting means for judging the direction of inhibiting the drive of the motor corresponding to the steering torque signal Vt and the motor current Im and outputting the rightward drive inhibition signal InhR and the leftward drive inhibition signal InhL.
FIG. 25 shows a characteristic of the drive inhibiting means in this conventional electric power steering apparatus in which characteristic of the drive inhibiting means is changed by the motor current.
Referring to FIG. 25, in the case that the motor current Im is larger than a threshold value Ith1, the characteristic is the same as that in the foregoing drive inhibiting means 4. However, in the case that the motor current Im is not larger than the threshold value Ith1, it is established that the drive inhibition signals InhR and InhL are authorized when the steering torque signal Vt is not more than a threshold value Tth2 and mot more than a threshold value xe2x88x92Tth2, i.e., when steering is not operated or steering is operated with a very small steering force.
In the conventional method for judging any failure in the drive inhibiting means 4 shown in FIG. 22, whether or not there is any failure is judged from the drive inhibition signals InhR and InhL regardless of the motor current Im. Accordingly, a problem exists in that, in case of combining the method with, for example, the drive inhibiting means 12 having the characteristic shown in FIG. 25, when the motor current Im is not more than the threshold value Ith1 and the steering torque signal Vt is not more than the threshold value Tth2, the drive inhibition signals InhR and InhL are authorized, and therefore it is mistakenly judged that there is a failure in the drive inhibiting means 12.
Another problem exists in that it is not possible to judge whether or not the characteristic of the drive inhibiting means 12 is appropriately switched corresponding to the motor current Im.
The present invention was made to solve the above-discussed problems and has an object of obtaining an electric power steering apparatus in which it is possible to detect a failure even if the apparatus is provided with drive inhibiting means whose characteristic is changed corresponding to a motor current Im.
An electric power steering apparatus according to the invention comprises: drive inhibiting means for outputting a drive inhibition signal to restrict drive of a motor according to a relation between a steering torque and a motor current; and failure detecting means for detecting a failure in the drive inhibiting means according to the relation between the steering torque and the motor current using the drive inhibition signal outputted by the drive inhibiting means.
As a result, it is possible to detect any failure in the drive inhibiting means restricting the drive of the motor according to the relation between the steering torque and the motor current.
It is also preferable that the drive inhibiting means outputs a drive inhibition switch signal corresponding to the relation between the steering torque and the motor current to the failure detecting means, and the failure detecting means detects a failure corresponding to the drive inhibition switch signal.
As a result, it is possible to detect a failure of the drive inhibition switch signal using the drive inhibition switch signal.
It is also preferable that the failure detecting means does not detect a failure when the drive inhibition switch signal and the drive inhibition signal satisfy predetermined conditions.
As a result, it is possible to simplify the failure detecting process.
It is also preferable that the failure detecting means does not detect a failure when the drive inhibition switch signal outputted by the drive inhibiting means is within a predetermined range.
As a result, it is possible to simplify the detecting process.
It is also preferable that the failure detecting means detects abnormality of the drive inhibition switch signal in association with the motor current.
As a result, it is possible to detect abnormality of the drive inhibition switch signal.
It is also preferable that the failure detecting means does not detect abnormality of the drive inhibition switch signal when the motor current is smaller than a predetermined value.
As a result, it is possible to simplify the failure detecting process.
It is also preferable that the failure detecting means does not detect a failure when the motor current is within a predetermined range near a switching point where the drive inhibition switch signal is inverted.
As a result, it is possible to reduce erroneous failure detection.
It is also preferable that the relation between the steering torque and the motor current is switched corresponding to the motor current, and the failure detecting means detects a failure corresponding to the motor current reflecting the state of switching the relation between the steering torque and the motor current.
As a result, it is possible to detect a failure in the drive inhibiting means using the motor current.
It is also preferable that the failure detecting means does not detect a failure when the motor current and the drive inhibition signal satisfy predetermined conditions.
As a result, it is possible to simplify the failure detecting process.
It is also preferable that the failure detecting means does not detect a failure when the motor current is within a predetermined range.
As a result, it is possible to simplify the failure detecting process.
It is also preferable that the relation between the steering torque and the motor current is established so as to authorize driving of the motor when the motor current is small.
As a result, it is possible to generate the steering assist force even if the motor current is small.
It is also preferable that the failure detecting means does not detect a failure when the steering torque is within a predetermined range.
As a result, it is possible to reduce erroneous failure detection.
It is also preferable that the failure detecting means reduces output of the motor when a failure is detected.
As a result, safety at the time of actually occurring a failure is assured.
It is also preferable that the failure detecting means concludes definitely that there is a failure when the failure is continuously detected for more than a certain time, and reduces output of the motor when occurrence of the failure is definitely concluded.
As a result, it is possible to prevent erroneous judgment that there is any failure due to noises and assure safety at the time of definite occurrence of the failure.
It is also preferable that the failure detecting means interrupts output of the motor when the motor current is reduced.
As a result, it is possible to prevent a rapid increase in steering force of the driver.
It is also preferable that the failure detecting means reduces gradually output of the motor.
As a result, safety of the apparatus is assured.
It is also preferable that the apparatus is provided with a memory for storing and holding results detected by the failure detecting means.
As a result, the apparatus becomes more highly reliable.
It is also preferable that the memory is a nonvolatile memory. As a result, it is possible to store the detection results even after interrupting the power source.